The present invention relates to a method for transmitting digital signals by means of signal generators in which a double pulse train is initially converted before transmission into a first single polarity, or unipolar, pulse train for all positive transmission pulses and into a second single polarity pulse train for all negative transmission pulses.
The simplest form of digital signal transmission is the noncoded binary signal transmission. One drawback of such a type of transmission is that it does not contain sufficient time information, i.e. it is impossible to synchronize the signal receiver when there are long periods when the signal value does not change. It is likewise impossible to monitor the signals for errors because there are no typical code characteristics which could be checked.
Frequently it is necessary to use a bipolar code for transmission, e.g. at interfaces in PCM networks, according to CCITT G. 703, the HDB3 code.
A bipolar code is less susceptible to interference since it does not contain a direct current component. However, when transmitting digital signals by means of optical fibers, the signal generator is known to be able to transmit only signals whose amplitudes are of identical phase, i.e. opposite signal amplitude polarities can not be distinguished. It would be conceivable to superpose the bipolar code on a direct current component which is so high that the negative and positive signal components would both have the same polarity with respect to the 0 level, i.e. would have identical amplitude phase values. But that would mean that the power density would increase considerably. Error correction and clock pulse recovery are preferably effected by means of an HDBn or CHDBn code, as those codes are defined by CCITT.
German Offenlegungsschrift [Laid-open Application] No. 2,844,293 discloses a method and system for modulation and demodulation for digital signal transmission in which the digital signal to be transmitted is coded according to a code which employs three signal images which differ from one another in time and lie within one signal period T, such as are obtained by modulating pulses of a carrier with respect to amplitude as well as position (pulse amplitude-pulse position modulation). This method requires a considerable amount of circuitry for coding and modulation. Moreover, it must then be determined in a decision circuit which one of the three signal images was transmitted. With a high data transmission rate and long transmission paths it may happen that delay errors occur which result in faulty scanned values during decoding
German Pat. No. 2,624,101 discloses a decoding method for an HDBn decoder in which a received pulse train in the HDBn code is initially converted into one unipolar pulse train for all positive transmitted pulses and into a second unipolar pulse train for all negative transmitted pulses. The conversion into these two pulse trains here serves only to permit examination of the pulses to monitor adherence to the HDBn rule and to monitor the transmission path and the decoder by testing the bipolarity. No mention is made in German Pat. No. 2,624,101 of digital signal transmission by processing these unipolar pulse trains or of an energy saving transmission method.
German Offenlegungsschrift No. 2,845,828 discloses a method for transmitting binary sequences wherein each bit of a first value is represented by a signal element of a first level during the first half of a bit clock pulse period and subsequently by a signal element of a second level which is clearly different from the first level and which occurs during the second half of the same bit clock pulse period. Here again, a complicated coding and decoding apparatus is required. Additional errors may occur due to the dependence of the code conversion on the clock pulse.